1. Field of the Invention
The present invention relates to a method of removing organic compounds from an air/permanent gas mixture, including conveying said air/permanent gas mixture (untreated medium) to a first gas separation membrane means and dividing said mixture into a gas stream (filtrate) that is concentrated with organic compounds and a gas stream (retained gas) that is depleted of organic compounds, with the concentrated filtrate gas stream being conveyed to a recovery device for the recovery of said organic. compounds therefrom.
2. Description of the Prior Art
In a known method of this type (U.S. Pat. 4,553,983 Baker dated Nov. 19, 1985 ), an exhaust air mixture that is to be separated, and that is at a high temperature, is conveyed to a furnace that is connected with a mixture-feed portion of the membrane means. The retained gas is again introduced into the furnace and is mixed with the exhaust air mixture that is to be separated and that is conveyed, as mentioned previously, to the furnace, and is again conveyed to the membrane means. The filtrate that is concentrated with organic compounds is conveyed to a compressor and subsequently to a recovery device, which is embodied as a condensor and from which the condensed organic compound exists. The exhaust air that leaves the recovery device is either conveyed directly to the atmosphere or is again conveyed to the furnace together with the retained gas coming from the gas separation membrane means.
Generally true is that the gas transport through a gas separation membrane is proportional to the transmembrane partial pressure difference. The separating capacity of a gas separation membrane, and hence the purity of the filtrate that is generated, is essentially a function of the pressure on the front side of the membrane to the filtrate pressure, i.e. the ratio of the retained gas pressure to the filtrate pressure. A gas separation membrane, in other words a method utilizing such a gas separation membrane, operates optimally only if not only a high inlet pressure as well as a pressure ratio adapted to the selectivity of the gas separation membrane can be established.
The drawback of this known method is essentially that these criteria can be fulfilled only at relatively low concentrations of organic compounds in air/permanent gas mixtures, so that this known method has the particular drawback that the exhaust air thereof still contains a high concentration of organic compounds. For example, this known method cannot maintain levels prescribed by regulations in the Federal Republic of Germany for the maximum organic compound content in the exhaust air of industrial plants.
Furthermore, there is a great need to provide a method with which, for example in connection with the continuously scarcer raw material resources on the one hand and the increased environmental awareness on the other hand, the fraction of the organic compounds from the considerable quantities of air/gaseous petroleum derivatives that form in petroleum tanks can again be condensed and utilized, and not be released into the environment.
It is therefore an object of the present invention to provide a method with which, without the need for much intrinsic energy, an optimum amount of organic compounds can be removed from an air/permanent gas mixture, with which the gas mixture discharged into the environment contains such a low proportion of organic compounds that prescribed values for foreign gas fraction can be maintained, with which little expense for apparatus is required, and which functions without difficulties eve when the concentrations of organic compounds in the air/permanent gas mixture (untreated medium) are high.